If you ever want to earn your Distinguished Rifleman badge, which I did, you end up spending every weekend at an Across the Course Match shooting around 88 rounds for the day. Even though that is less than many shooters do busting rocks in the quarry, the cumulative round count easily adds up to where you are replacing a barrel once a year, maybe in two years. Also, the people around you are shooting thousands of rounds in competition and you have this wonderful database of experiences to draw from, or add to.
Everyone who makes it to the Master Class or High Master level has heard of, or seen, or worse, had, a slamfire in a AR15. They typically occur in the standing stage when the muzzle is depressed and you are single loading. I had one with the new brass plated WSR's, which Winchester made more sensitive than the pre 2000 nickel plated WSR's. Sure scared the heck out of me, it was an in battery slamfire, during the standing stage and it took a divot out of the grass in front of the firing line. The bud on my firing point, when he was finished scoring me, he got up, loaded his AR15 single shot, and his rifle slamfired in battery. He was using Federal primers. I talked to him a number of years after that slamfire and he had another with Federals and was the last he used federal primers in his AR15. Federal now offers their mil spec version of their primer, which according to the ads, has a thicker cup. Both of us lower the bolt part way, in the standing position, before letting go of the charging handle. I also fire my sighting shots for rapid fire, from the magazine. I can't do that long range so I still toss a round in the chamber and hit the bolt release. I have never seen a AR15 slamfire in the prone position, so I think that little extra acceleration due to gravity makes a huge difference in the standing stage. Incidentally, the NRA used to let you load your rifle on the shooting stool. When Garands and M1a ruled the firing line that was not a problem, the muzzle was up when you loaded. However AR shooters put their muzzles on the stool, and hit the bolt release. Enough AR's slamfired through the shooting stool that the new rule was created for the safety of all competitors.
All fire arms with free floating firing pins are susceptible to slamfiring because primer sensitivity varies greatly. Commercial primers are made to go bang in everyone's mechanism, and there are a lot of mechanisms that have very weak ignition systems. Military firearms tend to have robust ignition systems, and military primers are less sensitive, on average, than commercial primers. However, slamfires with both types of primers occur, it is just the slamfires with military primers are less frequent. Never assume the rifle won't go off when loading, it might.
The military developed the #41 primer because early M16's were having slamfires. This is the appropriate primer to use in a AR15 type action as it has the lowest chance of a slamfire in this mechanism.
I found a 1960's test report where six M14's were being fired 6,000 rounds each. One slamfired out of battery with just a little more than 5,000 rounds through it. I am assuming the military slamfire probability for mil spec small rifle primers is the same for mil spec large rifle, so the chance of a slamfire each shot, with mil spec primers, is around 1:35,000. Commercial primers would be higher, maybe 1:3,000, maybe 1:10,000, hard to know without real test data.
CCI 400 primers are excellent primers, they seem to have thick cups, if mil spec primers are not available, I would use them over brass WSR and Federal match in AR's. I have not found the mil spec Feds, I would use those if I found a sheave cheap. I have shot HM scores all the way out to 600 yards with CCI #41's, they are an excellent primer, they can be hard to find, but they are my first choice in semi auto's.
Now I have some decisions to make and a lot of reading to do! Thank you all for the QUICK replies, opinions and links to some great articles!
